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Make DefId repr(C), optimize big-endian field order #92012

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Jan 11, 2022
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Make DefId repr(C), optimize big-endian field order #92012

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9 changes: 8 additions & 1 deletion compiler/rustc_span/src/def_id.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -221,10 +221,17 @@ impl<D: Decoder> Decodable<D> for DefIndex {
#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Copy)]
// On below-64 bit systems we can simply use the derived `Hash` impl
#[cfg_attr(not(target_pointer_width = "64"), derive(Hash))]
// Note that the order is essential here, see below why
#[repr(C)]
// We guarantee field order. Note that the order is essential here, see below why.
pub struct DefId {
// cfg-ing the order of fields so that the `DefIndex` which is high entropy always ends up in
// the lower bits no matter the endianness. This allows the compiler to turn that `Hash` impl
// into a direct call to 'u64::hash(_)`.
#[cfg(not(all(target_pointer_width = "64", target_endian = "big")))]
pub index: DefIndex,
pub krate: CrateNum,
#[cfg(all(target_pointer_width = "64", target_endian = "big"))]
pub index: DefIndex,
}

// On 64-bit systems, we can hash the whole `DefId` as one `u64` instead of two `u32`s. This
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